Carburetors for multi-purpose engines supply a considerably lower quantity of fuel to the engine in comparison with carburetors that supply fuel to four-stroke engines, such as automobile engines. Significant changes in fuel mixture ratio result from inaccuracies in carburetor component placement and dimension, and from deficiencies in component quality. These factors, along with the difference in performance associated with different engines, make it necessary to be able to adjust carburetor fuel flow quantity separately for each individual engine.
Given this necessity, a manually adjustable fuel valve is included in the design of some carburetors. Such valves comprise a needle-shaped, tapered valve inserted in the fuel jet of the carburetor and mounted on the end of a threaded rod which screws into the carburetor body. An extension at an opposite end from the needle valve extends from the threaded rod and protrudes from the carburetor body. By twisting the extension, the needle valve can be moved back and forth within the carburetor body, thus changing the effective cross-sectional area of the jet. This adjusts the quantity of fuel flow through the jet. Both a main fuel jet and a low-speed fuel jet can be equipped with such valves, thus making it possible to adjust fuel flow quantity separately for each jet.
These valves are normally adjusted by either the manufacturers of the carburetors, or the manufacturers of the engines, vehicles or appliances on which the carburetors are used, to ensure that the engine receives the proper amount of fuel. In certain situations, i.e., when the engine performance is affected by operating in different locations or under different conditions, or simply when there is a temporary loss of engine power, the user of the vehicle or appliance may attempt to adjust these valves. As a result, an excessively rich or excessively lean fuel and air mixture may be created, often resulting in loss of engine power, engine smiling, or poorer exhaust quality, i.e. more unwanted emissions.
In an attempt to reduce emissions from multi-purpose engines, regulations have been put into effect in recent years. These regulations make it necessary to equip these engines with a limiting device that allows the user to make adjustments, after the manufacturer has adjusted the carburetor, that result in emissions that are substantially only within a range allowed by the regulations. These devices must also be constructed such that they are difficult to remove from the carburetors.
Devices to limit the adjustment of the fuel adjustment valve have been described in the art. U.S. Pat. No. 3,618,906 describes a cap that has been installed on the end of the adjustment valve. The cap is equipped with a radially protruding appendage that limits adjustment to within one revolution because the appendage is obstructed by a stopper protruding from the carburetor body. U.S. Pat. No. 5,236,634 describes valves for both the main fuel jet and the low-speed fuel jet as being placed parallel and adjacent to each other and having a cap with an appendage being obstructed by the other adjustment valve, or cap thereon, acting as a stopper.
However, both of these valve adjustment limitation devices protrude from the carburetor body. Their exposure makes it easier for the user to remove them with a bit of ingenuity. Thus, these devices do not prevent deliberate and resolute tampering by the user.
Other shortcomings with these designs exist during the manufacturing or assembly process. It is a difficult task to assemble the very small parts by hand. In some instances the appendages are not positioned correctly in relation to their stoppers. This results in some carburetors having a wider adjustable range in one direction, which could possibly produce an excessively rich or excessively lean mixture and make it substantially possible to operate outside the legal limit for emissions. Additionally, the relatively large force needed to press the cap onto the end of a valve, presents a risk of possible engine damage or improper adjustment of the valve. Further, in some cases, it may be possible to override the radial lock between the cap and the adjustment valve.
Therefore, it would be desirable to have a limiting device, for a carburetor having manually adjustable valves that are able to adjust the effective cross-sectional area of the main and low-speed fuel jets separately, that are capable of preventing and revealing deliberate and resolute tampering by the user, eliminating the difficulty in handling small parts, and preventing the emissions, when the engine is being used in a normal manner, from exceeding the legal limitations due to an inaccurate setting made by the manufacturer. In addition, it would be desirable to have a limiting device capable of being installed adjustment valves with a relatively small amount of force.